Diverse populations of low molecular weight RNAs (1mwRNA), found in both prokaryotic and eukaryotic organisms, have been shown to play important roles in a wide variety of cellular processes including DNA replication, RNA processing, and mRNA translation. Their involvement in these processes affects the regulation of gene expression in normally growing and developing cells as well as diseased tissues and virus-infected organisms. Our laboratory is interested in the structure/function relationships of 1mwRNA sequences and the investigation of intermolecular 1mwRNA:RNA hybridization as an essential molecular mechanism in 1mwRNA function. Research efforts are presently focusing on mouse 4.5S hybridizing RNA (hybRNA), a new mouse 1mwRNA species of 87 nucleotides which is able to hybridize in vitro with messenger and 18S rRNAs. While the function of mouse 4.5S hybRNA is presently unknown, we believe it is likely to use its intermolecular RNA:RNA hybridization capabilities for cellular function(s). Initial work revealed its expression in all mouse tissues, indicating a required function in all cell types. More recent investigations have demonstrated the presence of 4.5S hybRNA-homologous gene sequences and/or 1mwRNA transcripts in bacteria, fungi, amphibia, and mammals. The occurrence of 4.5S hybRNA-homologous genes and 1mwRNA sequences in such evolutionary divergent organisms strongly argues for the conservation of this 1mwRNA species to carry out identical or similar biological functions essential in all organisms. The experiments of this investigation will examine both the cell biology and structure of mouse 4.5S hybRNA. Cell biology experiments examining its expression and cellular distribution under both normal and abnormal growth conditions will provide insight into the potential function(s) this 1mwRNA species plays in vivo. Structural studies defining the nucleotide sequences responsible for intermolecular RNA:RNA hybridization, as well as characterizing 4.5S hybRNA-binding proteins, will indicate nucleotide sequences/RNA structures/associated polypeptides of the 4.5S hybRNA molecule potentially importnat in those function(s). Analysis of experimental results obtained from both cell biology and structural investigations will lead to a better understanding of mouse 4.5S hybRNA structure/function relationships and the possible biological role(s) this evolutionaily- conserved 1mwRNA sequence play in all organisms.